How long until your money doubles?

What the Rule of 72 Calculator Does

This calculator estimates how many years it takes for an investment to double in value at a given annual rate of return. You enter the expected interest or growth rate, and it returns the approximate doubling time using the Rule of 72.

It is useful for investors, savers, and students who want a fast mental or on-screen check without building a full spreadsheet. It works for anything that grows at a steady compound rate: a stock portfolio, a savings account, an index fund, or even inflation eroding purchasing power.

How It Works: The Rule of 72 Formula

The Rule of 72 is a shortcut for compound growth. The formula is:

years to double = 72 / annual rate (in percent)

You can also flip it to solve for the rate you need to double in a target number of years: rate = 72 / years. The number 72 is used because it divides cleanly by 2, 3, 4, 6, 8, 9, and 12, making the math easy. It is an approximation of the exact compound-interest answer, which uses natural logarithms: ln(2) / ln(1 + rate).

Worked Example

Suppose you invest $10,000 at a 9% annual return. Divide 72 by 9: 72 / 9 = 8 years to double. So your $10,000 should grow to roughly $20,000 in about eight years.

Check it against the exact math: at 9% compounded annually, $10,000 reaches $19,925.63 after 8 years and $21,718.93 after 9 years, so the true doubling point is about 8.04 years. The Rule of 72 lands almost exactly right.

A second example: at 6%, 72 / 6 = 12 years. The exact figure is 11.9 years, again very close.

When the Approximation Is Most Accurate

The Rule of 72 is most accurate for rates between roughly 6% and 10%, where it tracks the exact answer within a fraction of a year. As rates move further from 8%, the error grows.

Some practitioners adjust the numerator for accuracy at different rates:

• Use 72 for typical rates around 6-10%.
• Use 70 for lower rates such as 2-3% (common for inflation or bonds).
• Use 73 or higher for rates above 15-20%, where 72 starts to understate the time.

Tips and Common Mistakes

Enter the rate as a whole number, not a decimal. Use 8 for 8%, not 0.08. Dividing 72 by 0.08 gives a meaningless answer.

Match the rate and the time period. The result is in years only if you used an annual rate. If you plug in a monthly rate, the answer comes out in months.

• The rule assumes a constant rate that compounds; real returns fluctuate, so treat the output as an estimate.
• It does not account for fees, taxes, or additional contributions, which all change real-world doubling time.
• It works the same way for decline: 72 / inflation rate tells you how fast prices double or money loses half its value.